The paper feeding system for band line impact printers comprises a pair of paper feeding tractors which engage pin feed holes at opposite edges of perforated paper and hold it in tension relative to the print forming elements of the printer. It is desirable that the tractors be movable separably to adjust for different widths of paper. It is also desirable that the tractors be movable together along the line of print for positioning the paper relative to the print elements. It is further desirable that the tractors be so movable by a mechanism which is simple to operate and which uses a single operating element such as a knob.
Mechanisms for moving tractors separably or jointly are of two general types, those using cables and those using lead screws. The invention relates to a tractor adjusting system using coaxial lead screws. Examples of tractor adjusting systems using coaxial lead screws for adjusting the spacing and position of paper feed tractors are shown in U.S. Pat. Nos. 3,578,138--Cantwell; 3,420,352--Moran et al and 3,006,520--House. A cable system for adjusting tractors with a single knob mechanism is shown in IBM Technical Disclosure Bulletin, Vol. 29 No. 12 of May 1987, pp. 5518-9. The mechanisms in all these cases have the disadvantages of having too many parts, of being complex and difficult to operate. In addition, they are not easy to assemble and disassemble when for instance it is necessary to repair or replace one of the tractor mechanism.